The entire disclosure of Japanese patent application No. 2000-304596 filed on Oct. 4, 2000 and Japanese patent application No. 2001-268898 filed on Sep. 5, 2001 including their specifications, claims, drawings, and summaries are entirely incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a data memory device, in particular to one having data memory elements with its direction of change in the data holding characteristic being different according to the value of data to be stored.
2. Description of the Prior Art
Ferroelectric memory elements using ferroelectric capacitors are known as data memory elements constituting data memory devices. FIG. 9 shows part of a circuit constitution of a type of conventional ferroelectric memory element, a ferroelectric memory element 2 of the 1T1C (one-transistor one-capacitor) type.
The ferroelectric memory element 2 comprises a ferroelectric capacitor 4 and a loading capacitor 6. The ferroelectric capacitor 4 has stored data of xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d in nonvolatile manner corresponding to the difference in residual polarization arising from hysteresis characteristic of the ferroelectric.
When reading data, a reading-purpose voltage is applied to the plate line PL to detect a voltage Vbl (not shown) appearing on the bit line BL. The value of the voltage Vbl varies with the value of the residual polarization of the ferroelectric capacitor 4.
Therefore, whether a data xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d has been stored in the ferroelectric capacitor 4 can be found by judging whether the voltage Vbl is higher or lower than a reference voltage Vref (not shown). Thus, nonvolatile data memory devices may be easily realized using the ferroelectric memory element 2.
However, the conventional ferroelectric memory element 2 has the following problems: When the same data are stored in the ferroelectric memory element 2 for an extended period of time, the hysteresis characteristic of the ferroelectric capacitor 4 is distorted by the so-called imprint effect, which is aggravated by repeated action of reading data from the ferroelectric memory element 2.
In particular in case of the so-called destructive type of memory element such as the ferroelectric memory element 2, data destroyed by reading must be restored by rewriting. The high voltage for the rewriting further intensifies the tendency of the imprint effect.
When the hysteresis characteristic of the ferroelectric capacitor 4 is distorted by the imprint effect, it becomes difficult to determine whether or not the voltage Vbl is greater than the reference voltage Vref and so data cannot be read accurately.
The present invention is intended to provide a data memory device capable of reading data accurately even after repeated reading actions by solving the above-described problems encountered with the conventional memory device.
A data memory device according to the present invention, comprising
data memory elements with its directions of change in the data holding characteristic being different according to the value of data to be stored,
a data memory element for storing data either as not reversed or as reversed in nonvolatile manner, and
a state memory section for storing whether the data stored in the data memory element is in the non-reversed state or in the reversed state,
characterized by being constructed such that, when reading data from the data memory element, the data stored in the data memory element is outputted either in non-reversed or reversed stats according to whether the value of the state memory section represents non-reversed or reversed state, the data stored in the data memory element is reversed and stored again in the data memory element, and the value of the state memory section is reversed and stored again.
While the present invention may be broadly described above, its constitution and contents together with its purpose and features will become more apparent from the following disclosure in reference to the appended drawings.